1. Field of the Invention
This invention lies in the field of electronic amplifiers and, more particularly, amplifiers suitable for low signal threshold amplification with low current drain operation.
2. Description of the Prior Art
There are a wide variety of present day applications which require amplification of low level signals under extremely low current drain or low power consumption conditions. For example, in the field of implantable electronic pacers for cardiac pacing, where the battery energy of an implanted pacer must be conserved to the maximum extent possible, there exists an acute need for an amplifier which is capable of detecting when low level incoming signals exceed a predetermined threshold and amplifying same so as to produce a resultant output pulse, under circuit conditions which require a minimum current drain from the pacer battery. More particularly, in demand-type pacers where output stimulus pulses are delivered only in the absence of natural pulses, the quiescent current drain from the battery is determined largely by the design of the amplifier portion which recognizes and amplifies the incoming natural beats and generates an output signal suitable for resetting of the pacer oscillator. For further background on this problem, reference is made to the co-pending application Ser. No. 608,465, filed Aug. 28, 1975, and titled "Multiple Function Demand Pacer With Low Current Drain" and assigned to the same assignee.
It has long been considered virtually impossible to design an amplifier suitable for use in an implanted pacer which would draw only on the order of 1 microamp, but this has remained a standard which the industry has hoped to achieve. At the same time, any successful amplifier design must provide that the characteristics will be essentially the same in the environment where the amplifier is to be used as they are when they are tested. In the specific example of electronic pacers, one of the critical requirements is that the drift in threshold level, or sensitivity, from the temperature at which tested as compared to the operational temperature of about 37.degree. C, be minimized to the greatest possible extent. Also, to insure uniformity of amplifier characteristics for production devices, it is desired that the amplifier design provide maximum independence of the normal variation in components, so as to avoid the increased expense of utilizing extremely low tolerance elements or difficult adjustment procedures.